Over a period of many years, the manufacture of pressure-sensitive adhesive tapes has been shifting from organic solvent systems to aqueous latices in order to reduce air pollution and fire hazard and also to eliminate the rising expense of organic solvents. Surfactants, wetting agents, or emulsifying agents are commonly used to protect the aqueous latex from coagulation, but as is pointed out in U.S. Pat. No. 3,551,479 (Emmons), "migration of the emulsifying agent to the surface of the film has a deleterious effect upon several properties such as adhesion . . . " (col. 2, lines 3-5). While the Emmons patent does not mention pressure-sensitive adhesives it is of interest for its suggestion of "using an emulsifying agent that is bound to the polymer . . . " (col. 1, lines 46-47).
U.S. Pat. No. 3,617,368 (Gibbs) concerns heat-sealable barrier coats made from an aqueous latex including an emulsifying agent that is bound to the polymer, called a "copolymerizable ionic material" (col. 3, line 3). The Gibbs patent suggests using "very small amounts of conventional wetting agents . . . to further aid in the attainment of particles of desired size" (col. 3, lines 48-55).
U.S. Pat. No. 3,770,708 (Knoepfel et al.) discloses pressure-sensitive adhesive tape, the adhesive layer of which is a copolymer of vinyl monomers such as a non-tertiary alkyl acrylate and zwitterionic monomer which in part serves as an emulsifying agent that is bound to the polymer. Such emulsifying agents are hereinafter referred to as "emulsifier monomers". All of the Knoepfel working examples also employ conventional emulsifying agents which do not become bound to the polymer and are hereinafter referred to as "external emulsifiers". The pressure-sensitive adhesive tape of the Knoepfel patent is characterized by "outstanding shear strength combined with excellent tack and peel strength without requiring cure of the adhesive" (col. 2, lines 12-16).
The pressure-sensitive adhesive tape of U.S. Pat. No. 3,922,464 (Silver et al.) is similar to that of the Knoepfel patent except in these respects: (1) it is removable, (2) from 0.2 to 5% by weight of at least one vinyl-unsaturated, homopolymerizable emulsifier monomer is copolymerized with the Knoepfel monomers, and (3) the zwitterionic monomer can be omitted. In regard to the third difference, the Silver patent states: "It is often desirable to incorporate zwitterion monomers into pressure-sensitive adhesives used in preparing tapes of the present invention. Inclusion of, e.g., 3-5% (by weight) or more of such monomers greatly improves the cohesive and shear properties of the adhesive, and even smaller amounts have beneficial effects" (Col. 4, lines 52-57). Of the Silver examples, only the tape of Example 2 omits the zwitterion emulsifier monomers. Recent tests of that tape show submarginal adhesion and cohesion for many uses.
Most of the zwitterion emulsifier monomers used in the Silver examples are not currently commercially available and tend to be expensive to make. At only the 2% weight level used in most of Silver examples, a zwitterion emulsifier monomer might raise the price of the tape above what potential users would be willing to pay.
The Silver adhesives also preferably employ "small amounts of external emulsifier, especially where short chain emulsifier monomers are used. Such emulsifiers, which are commonly employed in the preparation of many copolymer latices, assist in initial suspension of the monomeric reactants and tend to result in a more homogeneous appearing emulsion than when they are omitted." (Col. 8, lines 39-55).
U.S. Pat. Nos. 3,931,087 and 4,012,560 (Baatz et al.) concern pressure-sensitive adhesive tape based on an interpolymer of a specific sulfonic acid as an emulsifier monomer, an alkyl acrylate or methacrylate such as 2-ethylhexyl acrylate, and an optional third monomer such as vinyl acetate. An external emulsifier is necessary to control the colloidal particle size and to prevent instability and coagulation of the emulsion (paragraph bridging cols. 2 and 3 of both Baatz patents).
Members of another class of emulsifier monomers, i.e., sodium and/or potassium-p-styrene sulfonate, have been copolymerized with various alkyl acrylates, dienes, styrene, and fluorinated vinyl monomers to produce polymers which are said to have both improved heat resistance and tensile strength according to "Polymer Preprints", Vol. 24 (2), August 1983, pages 37-38, a publication of Division of Polymer Chemistry, Inc., American Chemical Society. Specifically reported are tests on copolymers of n-butyl acrylate and sodium and potassium salts of styrene sulfonate. No mention is made as to their utility.
U.S. Pat. No. 3,316,200 (Hatala) concerns aqueous paint vehicles and paints based on copolymers of terminally unsaturated vinyl monomer, preferably an acrylic polymer, and an emulsifier monomer which is there called an ionogenic unit. Among a large number of listed ionogenic units are sodium and potassium styrene sulfonate.
U.S. Pat. No. 4,387,172 (Agarwal et al.) describes emulsion type adhesive compositions such as may be obtained by copolymerizing a conjugated diene such as isoprene with a sulfonate containing monomer such as sodium styrene sulfonate. To the resultant latex is added a tackifier resin, and the emulsion is then doctor-bladed onto a substrate and dried to form a tacky adhesive film.